Effective Boolean Value (XQuery)
Applies to: 
SQL Server
These are the effective Boolean values:
False if the operand is an empty sequence or a Boolean false.
Otherwise, the value is true.
The effective Boolean value can be computed for expressions that return a single Boolean value, a node sequence, or an empty sequence. Note that the Boolean value is computed implicitly when the following types of expressions are processed:
Logical expressions
The not function
The WHERE clause of a FLWOR expression
Following is an example of an effective Boolean value. When the if expression is processed, the effective Boolean value of the condition is determined. Because /a[1] returns an empty sequence, the effective Boolean value is false. The result is returned as XML with one text node (false).
value is false
DECLARE @x XML
SET @x = '<b/>'
SELECT @x.query('if (/a[1]) then "true" else "false"')
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In the following example, the effective Boolean value is true, because the expression returns a nonempty sequence.
DECLARE @x XML
SET @x = '<a/>'
SELECT @x.query('if (/a[1]) then "true" else "false"')
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When querying typed xml columns or variables, you can have nodes of Boolean type. The data() in this case returns a Boolean value. If the query expression returns a Boolean true value, the effective Boolean value is true, as shown in the next example. The following is also illustrated in the example:
An XML schema collection is created. The element <b> in the collection is of Boolean type.
A typed xml variable is created and queried.
The expression
data(/b[1])returns a Boolean true value. Therefore, the effective Boolean value in this case is true.The expression
data(/b[2])returns a Boolean false value. Therefore, the effective Boolean value in this case is false.
CREATE XML SCHEMA COLLECTION SC AS '
<schema xmlns="http://www.w3.org/2001/XMLSchema">
<element name="s" type="string"/>
<element name="b" type="boolean"/>
</schema>'
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DECLARE @x XML(SC)
SET @x = '<b>true</b><b>false</b>'
SELECT @x.query('if (data(/b[1])) then "true" else "false"')
SELECT @x.query('if (data(/b[2])) then "true" else "false"')
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